1. Field of the Invention
The invention is related to the field of telecommunications, and in particular, to methods and systems for designing communication networks with wavelength converters.
2. Description of the Prior Art
Communication networks include network elements that are connected to each other through communication links. Some examples of network elements are transceivers, switches, routers, and cross-connect devices. Some of these network elements use optical technology to exchange communications over optical fiber links. One such technology is wavelength division multiplexing (WDM), where each wavelength of an optical signal carries a different channel of communications. Thus, an optical fiber link carries a number of channels of communications using WDM.
These optical transmission channels between two network elements are called lightpaths. A lightpath may travel through multiple fiber links. Some lightpaths operate at the rate of a few gigabits per second (Gbps). In some communication networks without wavelength converters, a lightpath occupies the same wavelength on all the fiber links that it traverses, which is referred to as wavelength continuity. When using lightpaths, the routing of the lightpaths and the wavelength assignment of lightpaths are determined, which is referred to as the “routing and wavelength assignment problem”.
In one example of the communication networks, the network elements within the communication network are connected in a ring configuration. Each network element is connected to two other network elements to form a ring. Some rings are connected to other rings, where shared network elements are stacked to interconnect rings. These ring configurations have poor scalability and use excessive resource redundancy. Thus, network configurations are migrating from ring networks to mesh networks, where network elements are connected to two or more network elements.
Network element or optical fiber link failure in a communication network results in loss of data or delays in data, which both result in revenue loss. There are two types of fault management to handle the failure of the network element or optical fiber link: protection and restoration. Restoration is a reactive procedure in which spare capacity is available after the fault's occurrence is utilized for rerouting the disrupted connections. Protection is a proactive procedure in which spare capacity is reserved during connection setup.
Protection fault management schemes are classified by type of rerouting and type of resource sharing. In a link-based rerouting, the connections are rerouted around the end network elements of the failed link. In a path-based rerouting, a backup path is selected between the end nodes of the primary path. In a dedicated resource protection, the network resources such as wavelengths are not shared between backup paths. In a shared resource protection, the backup paths do share resources such as wavelengths, which utilizes network resources more efficiently. Dedicated-path protection, shared-path protection, and shared-link protection are the major options for survivability in optical meshed WDM networks. Dedicated-path protection is also referred to as 1+1. In shared-path protection, the backup paths can share resources as long as the corresponding primary paths are not in the same shared risk group (SRG). Two primary paths are in the same SRG if they traverse the same fiber, or the fibers they traverse are in the same cable or in the same duct. In shared-link protection, the backup path for a link can share resources with the backup path for another link.
Mesh-based shared protection schemes take advantage of the mesh connectivity and achieve better resource utilization compared to 1+1. In a communication networks with wavelength converters, a lightpath does not have to occupy the same wavelength on all the links. Wavelength conversion facilitates the sharing among protection resources and improves the resource utilization in a network with shared protection.
Having wavelength-conversion at every node is not usually cost-effective. Next generation transport networks are expected to be hybrid, consisting of optical cross-connects (OXCs) of different architectures and technologies (OOO and OEO). OEO OXCs are capable of wavelength conversion, while OOO OXCs are not. Deploying all-optical wavelength-converters for every wavelength at every network node could be very costly. Choosing wavelength conversion sites is important to lower the overall network cost, which is called the Wavelength-Converter Placement (WCP) problem. Previous work has been focused on a network with dynamic traffic in which proper WCP can lower the call blocking probability. Prior work on WCP has not considered the protection schemes used in a network, which is an important element to network design.
The WCP problem is as follows. Given a network topology of N nodes and a set of traffic demands with a set of shared protection constraints, choose K wavelength-conversion sites (K<N) such that all the traffic demands are satisfied with a minimum network transport cost. Here the cost is measured in wavelength-links accumulated over all the working and protection paths. The converter placement not only affects the wavelength sharing among protection paths, but also affects the routing of working and protection paths. The WCP problem can be proved to be NP-hard, which means that, an optimal solution is not likely to be found if the problem size is big.